Nuclear waste is generally processed by heating liquid, slurry, and sludge wastes to dryness, oxidation/destruction of organic materials contained therein followed by further heating, using microwaves, to yield oxides of the radioactive elements and other metals present. The oxides fraction may then be extracted in-situ into glass/ceramic matrices and immobilized by continued microwave heating in the presence of glass-fritts and ceramic former additives in the same reactor vessel at temperatures 600.degree. C.-1450.degree. C. Suitable waste forms are borosilicate glass and ceramics (synrocs or mixed zirconia/alumina-based ceramic calcines mixed with a fritt composition). The final product includes glass/ceramic monolith waste forms that can be safely disposed of. High-temperature thermal "melter" furnace/incinerator combinations are inefficient and unsafe, since they require removal of the wastes from their containers and because thermal heating is non-selective.
Microwave heating has many advantages over other forms of heating. For example, microwave heating is material-selective and provides bulk heating of the material from the inside out; it is rapid when there exists an efficient coupling (high-susceptiblity) of microwave energy into the material; and it can be applied to dangerous materials that are prone to become airborne, since microwave reactor systems can be operated as enclosed systems. Microwave energy interacts with materials by inducing rotations in molecules or in ion-pair dipoles, with subsequent conversion to heat. The microwave power, P, dissipated in bulk material as heat is given by the relationship, P .pi..nu..epsilon.'E.sup.2 tan.delta. (W/m.sup.3), where, E= electric field intensity at microwave frequency, .nu., .epsilon.' is the dielectric constant, and tan.delta.=.epsilon."/.epsilon.', where .epsilon..epsilon. is the dielectric loss constant.
Microwave processing technology is employed in the processing of nuclear and medical wastes. Much of this waste is stored in sealed drums for which heat processing has many advantages. For example, nuclear wastes can be calcined, sintered, or melted in the presence of ceramic precursors or glass fritt additives in the drums at temperatures .about.1200.degree. C. or higher at ambient pressure in order to immobilize the radionuclides from the waste in ceramic or glass matrices. In some instances, the material may be contaminated with harmful organic molecules which are destroyed by in-situ air oxidation in the same reactor prior to proceeding with the immobilization of the radioactive components. The resulting sintered materials are mechanically durable and non-leachable.
However, currently employed microwave processing technology is unable to provide uniform heating in materials to be heat-processed in the temperature range between 600 and 1450.degree. C. in cylindrical cavity reactors, such as drum cavity reactors. For example, TE.sub.10 -mode microwave radiation (0.915 GHz, 50 kW) propagating through a rectangular WR975 waveguide and coupled through an adjustable iris plate interface located between the waveguide and the open end of a 55 gallon drum, is transformed essentially into a single, TE.sub.11 cylindrical mode in the cylindrical drum. This mode deposits significant microwave energy near the drum center, but substantially less energy near the cylindrical outer wall of the drum. Powdered materials processed in such drum reactors have been observed to possess a monolithic structure near the center of the drum, while the powder near the drum wall was unaltered.
Accordingly, it is an object of the present invention to provide an apparatus for uniform heating of materials in cylindrical containers, 10-55 gallon drums, for example, using microwave radiation.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.